The 186 Ma Dashibalbar alkaline granitoid pluton in the north-Gobi Rift of central Mongolia: Evidence for melting of Neoproterozoic basement above a plume
Bibliographic record
Abstract
The Jurassic Dashibalbar granitoid pluton (∼300 km^2^) crops out in the Triassic North-Gobi rift of central Mongolia, just south of the 230 to 195 Ma Khentei batholith. The granitoids are shallow-seated dominantly, amphibole-bearing alkali feldspar granite that contain quartz-syenite/syenite enclaves. They are all composed of megacrystic mesoperthite, quartz, Ca-Na amphibole altered to biotite and rarely with pyroxene cores, magnetite and ilmenite. The pluton yielded a concordant U-Pb zircon age of 186 ± 1 Ma, which is similar to a published 189 ± 3 Ma ^40^Ar/^39^Ar amphibole age, and indicates rapid cooling through *ca.* 550 °C. This age is *ca.* 10 my younger than the 196 ± 4 Ma age of the bimodal volcanic complex intruded by the pluton. The volcanic complex is composed of augite-phyric transitional basalt and rhyolite/comendite. Both basalts and rhyolites/comendites are evolved within-plate varieties with positive ε~Nd(t)~ (∼ +2.5) values. The granitoids are evolved alkaline, A-type granites and quartz-syenites/syenites that are enriched in light REE9s, but show a distinct depletion in Eu, Sr and Ba, indicative of feldspar fractionation. The data are consistent with derivation of the granites from the syenites by an assimilation-fractional crystallization process involving a silicic crustal contaminant. The granitic rocks have ε~Nd(t)~ values of ∼ +0.8 to +1.2, which are slightly lower than ε~Nd(t)~ values +1.3 to +1.6 in the syenites, although both have similar T~DM~ model ages (∼800-970 Ma). The ∼800 Ma model ages of the basalt and rhyolite/comendite are comparable to those of the intrusion and enclaves. The compositions of all these rocks, including ε~Nd(t)~ and T~DM~, are within the range of A-type granites and volcanic complexes of the Early Mesozoic Mongolian-Transbaikalian igneous province. The results suggest derivation of a parent magma of the granitoids and felsic volcanic rocks from underplated, enriched, Neoproterozoic mantle-derived basaltic rocks in the lower crust, whereas the Dashibalbar basalts were derived from Neoproterozoic subcontinental lithospheric mantle; Neoproterozoic megablocks crop out in adjacent parts of the Central Asian Orogenic Belt. Melting of lower crust and subcontinental lithospheric mantle implies a rising heat source. Although such a heat source is consistent with both rifting and passage over the Mongolian mantle plume, only the latter explains the west-to-east migration of the magmatism and rifting.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".